1. Field of the Invention
The present invention relates to a thin film transistor and a display device, and particularly to an oxide semiconductor thin. film transistor which uses an oxide semiconductor for a. semiconductor layer and a display device using the same.
2. Description of the Related Art
There is an oxide semiconductor thin film transistor (also referred to as an oxide thin film transistor or an oxide semiconductor TFT) as a favorable thin film transistor in which the mobility is high and a variation in a threshold voltage Vth is small in an approximately equivalent process as compared with an amorphous silicon thin film transistor (Si-TFT). When the oxide semiconductor TFT is implemented, a high density silicon oxide film is used for a channel protective layer (stopper layer), and thereby it is possible to implement a thin film transistor with good reliability, high mobility, and high On/Off ratio. However, if this high density silicon oxide film is to be formed thickly, a film stress increases and thus a problem such as film peeling occurs, and thereby reliability of a thin film transistor is reduced.
As a method of solving the reduction in the reliability, there is a thin film transistor disclosed in JP 2010-135462 A or JP 2009-272421 A. In the thin film transistor disclosed in JP 2010-135462 A, a first channel protective layer and a second channel protective layer are formed on an oxide semiconductor layer, and either one of the channel protective layers is made of a material with low oxygen permeability. In this case, the first channel protective layer is made of silicon oxide, tantalum oxide, titanium oxide, hafnium oxide, zirconium oxide, yttrium oxide, aluminum oxide, or a nitrogen-containing compound thereof, or silicon nitride.
In addition, in the thin film transistor disclosed in JP 2009-272427 A, a channel protective layer is formed of two layers including a first channel protective layer and a second channel protective layer, and the second channel protective layer is laminated on the first channel protective layer. In this case, the channel protective layer is formed of an oxide such as a silicon oxide film, a silicon oxynitride film, an aluminum oxide film, an yttrium oxide film, or a magnesium oxide film.